FIG. 2 shows a whole sectional diagram of a conventional hot wire air flow meter. Referring to FIG. 2, a heating resistor 1 and an intake air temperature detecting resistor 2 having the same structure as the heating resister are arranged within a bypass passage 4 of a body 5 that has a main passage 3 in which a major portion of the intake air passes and the bypass passage 4 in which a portion of a divided intake air passes.
FIG. 3 shows a driving circuit 6 of the hot wire air flow meter. The driving circuit comprises the heating resistor 1, the intake air temperature detecting resistor 2, operational amplifiers 7 and 8, a power transistor 9, a condenser 10, and resistor 11-15. A corrector terminal 16 of the power transistor 9 is connected to (+) pole of a battery, an earth 17 of the resistor 1 is connected to (-) pole of the battery, and a connection point of the resistor 11 and the heating resistor 1 is connected to an input terminal 18 of a micro computer which carries out an engine control using the output signal of the hot wire air flow meter.
In such a structure, the temperature of the heating resistor 1 is controlled to be higher than that of the intake air temperature detecting resistor 2 by a constant temperature by supplying a current to the heating resistor 1. At this time, the intake air temperature detecting resistor 2 is used for compensating the temperature of the intake air in such a manner that the heating of the intake air temperature detecting resistor is neglected and the intake air temperature is able to be detected. When air blows against the heating resistor 1, the temperature difference between the heating resistor 1 and the intake air temperature detecting resistor 2 is controlled to be constant by the operation of the driving circuit 6 as explained above. The control is carried out by feedback in such a manner that a divided voltage of a voltage difference of both ends of the heating resistor 1 by the resistors 12 and 13, and a voltage, in which a voltage drop of the resistor 11 caused by the current flowing through the heating resistor 1 is amplified by the operation amplifier 8, are always equal to each other. Accordingly, when the mass air flow changes, the current flowing through the heating resistor 1 is changed so that the air flow is able to be measured by the voltage drop of the resistor 11 corresponding to the current. By inputting the measured signal of the intake mass air flow of an automobile engine to the micro computer through the terminal 18, the micro computer calculates the necessary fuel volume for maintaining an optimum combustion so that an electronic fuel injection system which is able to inject fuel by an injector can be used.
FIG. 4 shows one conventional example of the heating resistor which is used for such a hot wire air flow meter. The heating resistor has a pin 22 made of Pt-Ir (10%) inserted into respective ends of a bobbin 21 made of alumina, and a platinum wire 23 is wound thereon. Both ends of the pin 22 are supported on a supporting conductor 25 by welding 24.
In another prior art, construction as shown in FIG. 2 of Japanese Patent Laid-Open No. 58-95265, there is an example in which the resistor element is directly brazed to the supporting conductor 25, without using the pin 22.
However, these prior efforts have a draw-back in that response time of the measuring apparatus is bad, as shown by the dash line curve of FIG. 5; and when the mass air flow changes from Q.sub.1 to Q.sub.2 in a constant time, the value of the output voltage (V) does not rise sharply, but rises along a gentle slope curve. The drawback causes a big problem, when the hot wire air flow meter is used for an electronic control fuel injection apparatus of an internal combustion engine, such as an automobile engine.